The mouse Lyt-2 antigen is a T-cell marker, expressed on cytotoxic and suppressor (but not helper) T-cells. The importance of this surface antigen in T-cell functions is suggested by the finding that antibodies to Lyt-2 can block T-cell cytotoxicity in the absence of complement. In the present study, we have made use of monoclonal antibodies to determine the subunit structure of this antigen in thymocytes and in T-cell clones, and have entertained the possibility that it may be involved in the construction of one class of T-cell receptors.